The ink consumption in a modern, high speed printing press is very large and as a result, ink is frequently pumped to the press in pipes installed for that purpose.
Most offset printing inks are quite viscous, and are comparable to very thick molasses in this respect. As a result, ink neither flows readily, nor seeks its own level quickly. When the valve in the ink line is opened to introduce additional viscous ink into an ink fountain used in a printing press, the new ink takes a long time to level out.
Ink generally is considered an active substance in the sense that the ink sets when allowed to stand and dries when exposed to air. In addition, ink is sticky and readily adheres to all commonly used materials. These properties pose a difficulty in sensing ink level since the ink has a tendency to accumulate or build up on any sensing member or component with which the ink comes into contact.
Typically, the ink fountain or reservoir in an offset printing press is constructed in the form of a trough such as shown in U.S. Pat. No. 3,848,529 to Gegenheimer et al, which is assigned to the same assignee of this invention. The ink trough is formed by a flat blade which forms a nip with a fountain roller. A feature of such reservoir is that the surface of the ink contained therein is not flat, but usually undulates while the press is in operation at a frequency in the range from 0.25 to once a second. This undulation is produced by a ratchet action of a fountain roller and makes it difficult to sense the ink level.
Ink fountains in many printing presses are commonly equipped with ink agitators which improve ink distribution, prevent the formation of surface skin and improve feeding of the ink to the nip between the flat blade and the fountain roller. The main element of an ink agitator may be in the form of a conically shaped roller which is caused to rotate while traveling to and fro the length of the ink fountain. Such roller is illustrated in the aforementioned Gegenheimer et al patent. The presence of an agitator produces a wave in the ink of a height which may be of the order of an inch or even more. When the ink is so disturbed, its surface quivers and undulates so as to make it difficult to sense ink level.
The space above the ink fountain must be kept relatively clear to allow room for the operator of the printing press to clean the fountain or to enable ink to be fed manually for occasions when special or spot colors are necessary. As a result, the sensing of ink level must be done in such manner that the area or space over the ink fountain is not unreasonably obstructed.
When multi-color printing presses having many ink fountains are involved, a plurality of ink level control systems are employed. On any particular printing operation, however, it may not be necessary to use all of the ink fountains. As a result, some of the unused fountains may not have any ink and the ink level control must be able to sense such condition to avoid system malfunctioning.
A large number of automatic ink level sensor designs have been proposed. Ink level sensors may use floating, tactile, capacitance, pneumatic or ultrasonic techniques. U.S. Pat. No. 3,025,793 describes an ink level control for a newspaper press utilizing a float operated ink feed valve. Although such an approach is workable in newspaper presses, which utilize relatively thin or low viscosity inks, the float concept is not feasible in commercial presses requiring thicker inks. A major difficulty encountered with a floating device resides in the accumulation of ink on the float, eventually rendering it inoperable. The float also tends to take up a large amount of space, particularly when such float is directly connected to operate an ink flow control valve.
The U.S. Pat. No. 3,373,052 and the previously mentioned Gegenheimer et al patent are examples of ink level controls wherein a tactile type sensor detects the ink level by touching the ink. Such sensor does not circumvent the difficulties due to ink activity and, as a result, the sensor tactile element must be periodically cleaned for reliable operation.
In the aforementioned Gegenheimer et al patent, a tactile sensor is employed to monitor the height of the wave of ink generated by an agitator, thereby overcoming the difficulty posed by the waves in measuring the ink level. Gegenheimer et al also discloses the use of an adjustable timer to limit the duration of ink feed to overcome the difficulty posed by the high ink viscosity which prevents an even ink level from being quickly established when the ink valve is opened. In one form for such timer, adjustable feed pulses are applied through a relay to actuate the ink valve. The width of the feed pulses and the intervals between the pulses can be selected.
A number of prior art systems employ sensors which avoid ink contact by using the capacitance principle. In one approach, such as disclosed in the U.S. Pat. No. 4,010,683, a plate is mounted above the ink fountain to form an electrical capacitor with the ink fountain blade. Since the permitivity of ink differs significantly from air, the capacitance will vary with ink level and thus provides a non-contacting method for detecting ink level. This sensor design has a disadvantage in that the sensor must be located within approximately an inch of the ink surface, thus obstructing access to the fountain.
A pneumatic ink level measuring system is available in the form of a vertical tube which is inserted into the ink fountain. The tube is connected to a source of air and the pressure in the tube is monitored to obtain a measure of ink level. This concept is satisfactory for low viscosity inks but difficult to apply to highly viscous offset printing press inks.
Ultrasonic techniques for detecting the level of liquids are well known and have been applied to ink level controls in a limited liquid contacting manner. A general description of ultrasonic techniques can be found in an article entitled "Ultrasonic Instruments for Level and Flow" in the September, 1974 issue of Instrumentation Technology.
Ultrasonic systems for determining liquid level utilize one of two basic approaches. In a first liquid contacting type, a detector provides an on-off signal when the liquid comes into contact with the ultrasonic sensor; see, for example, U.S. Pat. No. 3,520,186 to Adams et al. The liquid level may interrupt ultrasonic waves or cause a change in the dampening characteristics of an ultrasonic transducer when it is contacted by the liquid. Contact by the detector with ink, however, is not desirable.
In another type of ultrasonic level detector, an echo ranging principle is employed, such as described in the U.S. Pat. No. 3,985,030 to Charlton. A pulse of ultrasonic energy is directed to a transducer toward the liquid surface. A receiver listens for an acoustic echo and the time required for the echo to return provides a measure of the distance between the ultrasonic transducer and the liquid surface. In the Charlton patent, a loss of echoes is detected and used to override an automatic level indicator by registering a maximum liquid level depth.